This invention relates to a process for removing hydrocarbons from a solution. More particularly, this invention relates to a process for absorbing hydrocarbons from a solution onto a medium and to the in situ regeneration of the absorbing medium. Typically, the solutions are composed of polar solvents, such as alkanolamines, glycols, sulfolane, and their aqueous mixtures.
The removal of acid gases (such as H2S and CO2) from natural gases, enhanced oil recovery gases, petroleum gases and liquids, tail gases, ammonia plant gases, coke oven gases, and the like, is commonly accomplished by alkanolamine sweetening units. When alkanolamine amine solutions are used to absorb acid gases from gas and liquid streams, hydrocarbons may also be absorbed or entrained in the alkanolamine solution.
The aqueous solutions of alkanolamines are contacted with streams containing H2S and CO2, the H2S and CO2 dissolve into the alkanolamine solution to form solvated alkanolamine salts (e.g., protonated alkanolamine cation with HSxe2x88x92 and HCO3xe2x88x92 anions). The solution of water, unreacted alkanolamine, and alkanolamine salts is subjected to steam stripping to decompose the alkanolamine salts and remove H2S and CO2 from the alkanolamine. Thiocyanate anion (SCNxe2x88x92), for example, forms from the reaction of HCN with H2S or sulfides in the gases from crude units or catalytic reformers. Other typical alkanolamine salt anions include S2 O32xe2x88x92, SO32xe2x88x92, SO42xe2x88x92, HCO2xe2x88x92, CH3CO2xe2x88x92, and the like. These alkanolamine salts cannot be removed by steam stripping. Thus, they are called heat stable salts and remain in the system where they accumulate in the alkanolamine solution, gradually depleting the effectiveness of alkanolamine treatment.
Generally, the heat stable salt anions are removed by exchange with hydroxide from an anion exchange resin and cations, such as sodium and potassium, are removed by exchange with hydrogen ion from a cation exchange resin. Ion exchange to remove heat stable salts from alkanolamine solutions has been a subject in the patent literature for many years, see for example, Taylor, U.S. Pat. No. 2,797,188; Pearce, U.S. Pat. No. 4,477,419; Keller, U.S. Pat. No. 4,970,344; Yan, U.S. Pat. No. 4,795,565; Veatch, U.S. Pat. No. 5,006,258; and Cummings, U.S. Pat. No. 5,162,084.
Unfortunately, hydrocarbons also cause problems for alkanolamine system operations even at very low concentrations (i.e., cleaning natural gas). Scrubbing efficiency and stripping efficiency are reduced, foaming can cause upsets and solvent loss, production may be curtailed, and environmental discharge limits may be exceeded. The situation is mirrored in other solvent extraction processes that employ hydrophilic solvents such as glycol dehydrators. Current technology relies on absorption onto activated carbon granules to remove hydrocarbons from amine systems. For example, U.S. Pat. No. 2,762,852, is directed to an adsorption process in which a hydrocarbon is contacted with activated carbon. When saturated, the activated carbon must be separately regenerated or exchanged for new carbon. Thus activated carbon adsorption is expensive and labor intensive.
In another type of process, U.S. Pat. No. 2,963,519 uses a zeolitic molecular sieve as the adsorbent to remove small diameter hydrocarbons from petroleum refinery hydrocarbon streams. The molecular sieves are regenerated by heating and passing hot adsorbate through the bed.
In another process, that disclosed in German Patent No. 155,036, aromatics are removed from non-aqueous hydrocarbon mixtures by adsorption onto a cation ion-exchange resin that has been charged with silver ions. Adsorbed aromatics are desorbed by flowing water through the resin, then the adsorbed water is removed from the resin by evaporation or by flushing with a volatile solvent.
In yet another type of process, U.S. Pat. No. 2,718,489 to Coonradt et al., describes removing ionic mercaptans from non-aqueous hydrocarbon mixtures using ion exchange resins. After the ion exchange capacity of the resins is consumed, typical ionic regeneration of the resin is done. This patent has for its purpose a method of activating fresh or regenerated ion-exchange resin by contacting with one or a mixture of lower molecular weight monohydric alcohols prior to cleansing a hydrocarbon fluid. The activation is said to improve the effectiveness of the resin at removing mercaptans from hydrocarbon liquids.
Another type of process, U.S. Pat. No. 4,775,475 to Johnson, describes removing hydrocarbons from an aqueous feed stream by contacting the stream with an adsorbent whereby the spent adsorbent is regenerated by means of an elution solvent. Examples of suitable adsorbents include molecular sieves, amorphous silica-alumina gel, silica gel, activated carbon, activated alumina and clays. Examples of elution solvents include naphtha, kerosene, diesel fuel and gas oil.
It has been discovered that hydrocarbon impurities can be removed from hydrophillic solvent solutions by means of water-wet resins and that these resins can be regenerated without the use of hydrocarbon solvents.
It is, therefore, the general object of the present invention to provide a process for absorbing hydrocarbons from an alkanolamine-, glycol-, or other polar solvent-containing solution onto an ion exchange resin and to the in situ regeneration of the ion exchange resin.
Another object of this invention is to provide a process for regenerating a filter bed of ion exchange resin having a hydrocarbon adsorbed thereon.
In accordance with the present invention, there is provided a process for removing hydrocarbons from liquid streams such as an amine-containing solution or a glycol-containing solution by passing the solution through an ion exchange resin in a filtration bed, whereby the hydrocarbons are retained by the bed. The resin is either cation or anion exchange resin. The liquid stream passes through the ion exchange resin and leaves the filtration bed substantially reduced in hydrocarbon content.
The ion exchange resin may be regenerated in situ by removing the hydrocarbons from the resin bed by flowing water therethrough, such that the hydrocarbon is carried away in that liquid. The ion exchange resin may be reused to remove hydrocarbons from a liquid stream.